Don't use deprecated isMounted anymore

Summary:
Even though this is a mixin, which is already pretty darn deprecated, the new `create-react-class` package gives //extra// warnings about calling `this.isMounted()`.

In this change, we remove `this.isMounted()` from `BackboneMixin`, and reimplement mount state ourselves. This is just as bad, but is a quick way of suppressing the error message that's crashing my unit tests :/

Test Plan:
In the Khan Academy webapp:

```
tools/tsm-2017-js-codemods/upgrade_js.py javascript/projectfeedback-package/projectfeedback-form.jsx
tools/runjstests.py javascript/projectfeedback-package/projectfeedback-form_test.jsx
```

Before this change, this test fails, with a "isMounted is deprecated" error message. After this change, this test passes.

Auditors: john

Author: matchu

js/backbone-mixin.jsx

@@ -26,14 +26,20 @@
  * This binds *and* unbinds the events.
  */
 const BackboneMixin = {
+    _isMounted: false,
+
     componentDidMount: function() {
+        this._isMounted = true;
+
         this._backboneModels = this.getBackboneModels();
         this._validateModelArray(this._backboneModels);
 
         this._bind(this._backboneModels);
     },
 
     componentWillUnmount: function() {
+        this._isMounted = false;
+
         this._unbind(this._backboneModels);
     },
 
@@ -79,7 +85,7 @@ const BackboneMixin = {
         // TODO(joel): more rigorous fix needed? -- for the following error:
         // "Invariant Violation: forceUpdate(...): Can only force an update on
         // mounted or mounting components."
-        if (this.isMounted()) {
+        if (this._isMounted) {
             this.forceUpdate();
         }
     },